FIG. 1 illustrates, by example, a block diagram of a conventional operational amplifier 100 (hereinafter referred to as "amplifier") and a load 110. The amplifier 100 is a basic analog building block for may types of analog signal processing applications. These applications may include active filtering, signal amplification, and voltage regulation to name a few.
The amplifier 100 of FIG. 1 generally includes a transconductance stage 101, a compensation network 102 and an output driver 103. FIG. 1 also illustrates an output load 110.
The interconnection of the blocks of the amplifier 100 and operation thereof is described as follows. The transconductance stage 101 is coupled to receive input signal 105 and input signal 106 and operative to produce an amplified signal at line 107. The compensation network 102 is operative to frequency compensate the amplified signal at line 107. The compensation network 102 may optionally be coupled to receive an output signal at line 109. The output driver 103 is coupled to receive amplified signal 107 and operative to produce output signal 108 to drive the output load 110.
The operation of the amplifier 100 is described as follows. A differential input signal between input 105 and input 106 is amplified in transconductance stage 101 and converted to a current signal at line 107. The compensation network 102 provides frequency compensation of the amplified current at line 107 by converting the current signal to a voltage signal by way of the impedance it provides at line 107. The output driver 103 provides voltage and/or current gain to the amplified signal at line 107 and produces the output signal 108 which appears across the load 110. The load 110 may have a complex as well as a real impedance associated with it.
Amplifiers may be configured as open or closed loops. Amplifiers have gain defined by the relationship between the amplifier's input and output. The gain has frequency dependent amplitude and phase. In order for the amplifier to be unconditionally stable in a closed loop configuration, the amplitude of the open loop gain versus frequency must drop below unity before the open loop phase exceeds 180 degrees. Phase margin is defined as 180 degrees minus the open loop phase at the frequency at which the amplitude of the open loop gain drops below unity. A phase margin of 45 degrees is usually considered adequate. It is often desirable for the phase margin to be at least 45 degrees over a wide range of load impedance's.
A problem with the conventional amplifier 100 is that the output driver 103 may be a physically large device with associated large input capacitance. The large input capacitance appears in parallel with the compensation network 102 and can severely alter its desired frequency compensation characteristic.
Another problem with the conventional amplifier 100 is that in some applications, such as voltage regulators, the complex impedance associated with the output load may be significant. This complex impedance may add additional phase shift to the open loop gain and degrade the phase margin of the amplifier. If the performance of the compensation network 102 is severely altered by the output driver's input capacitance, then the range of values for acceptable load impedances may have to be severely restricted in order to guarantee sufficient phase margin for stable operation.
A solution to these problems involves reducing the bandwidth of the amplifier by reducing its unity gain open loop frequency. However, a disadvantage of this solution is that the amplifier 100 can't amplify high frequency signals because of the reduced bandwidth. Further, reducing the bandwidth may require physically large components such as capacitors. Still further, the amplifier 100 is susceptible to high frequency noise on the power supply or ground to the amplifier.
Accordingly there is a need for an apparatus and method for frequency compensating an operational amplifier advantageously providing wider bandwidth and improved stability with complex low impedance loads.